fossilize: postject strips Mach-O signature on inject — but explicit unsign is still required

Trap: postject accepts a signed Mach-O and strips the signature itself during injection, so it looks like you can skip the unsign step. Fix: still call unsignBinaryInPlace() explicitly before postject, because Node's documented SEA flow is unsign → inject → sign and relying on postject's side-effect is fragile. Also: getNodeBinary writes darwin/win binaries at 0o644 (no execute bit) — Linux's copyFile preserves 0o755. Running the binary directly before an explicit chmod 0o755 fails with EACCES on macOS.